Mechanically tunable superconducting qubit

ABSTRACT

A system for adjusting qubit frequency includes a qubit device having a Josephson junction and a shunt capacitor coupled to electrodes of the Josephson junction. A cantilevered conductor is separated from the shunt capacitor by a spacing. An adjustment mechanism is configured to deflect the cantilevered conductor to tune a qubit frequency for the qubit device.

BACKGROUND

Technical Field

The present invention relates to superconductor qubits, and moreparticularly to devices and methods for tuning qubit frequencies.

Description of the Related Art

Superconducting quantum bits (qubits) show great promise for quantumcomputing, with ever-increasing coherence times combined with thepossibility of standard semiconductor fabrication methods leading to auseful quantum computer. However, one outstanding problem is in thecontrol of qubit frequencies.

Quantum computers will require qubits with tightly controlleddistributions of frequency. For the specific case of superconductingqubits, the frequency is determined by a Josephson junction criticalcurrent I_(c) and a total shunt capacitance, with a frequency given byf=½π√{square root over (LC)}. The inductance L is dominated by that ofthe Josephson junction and is given by

$L = \frac{\Phi_{0}}{2\pi\; I_{c}}$where Φ₀ is the flux quantum. The capacitance C is the sum of junctioncapacitance and the shunt capacitance. Typically, experiments andestimates both show that the capacitance can be well controlled andtypically fluctuations are due to variations in junction criticalcurrent. Typical distributions of frequency have standard deviations of5-10%. It is desirable to control the frequency of the qubit populationto much better than this, typically providing standard deviations ofless than 1%.

The observed spread in Josephson junction critical currents is in therange of 10% standard deviation, corresponding to ˜5% spread in qubitfrequency. Despite decades of research into fabrication and control ofJosephson junctions, no significant improvement has been seen in thespread observed. Similar spreads are observed in other oxide junctiontechnologies such as magnetic memory (MRAM), indicating that this may bean intrinsic property of such devices.

Qubit frequencies need to be controlled, and the fabrication processtypically used for making these devices results in a spread offrequencies, which is insufficient for application in a large scalequantum computer.

SUMMARY

A system for adjusting qubit frequency includes a qubit device having aJosephson junction and a shunt capacitor coupled to electrodes of theJosephson junction. A cantilevered conductor is separated from the shuntcapacitor by a spacing. An adjustment mechanism is configured to deflectthe cantilevered conductor to tune a qubit frequency for the qubitdevice.

Another system for adjusting qubit frequency includes a qubit deviceincluding a Josephson junction and a shunt capacitor coupled toelectrodes of the Josephson junction. A housing at least partiallyencapsulates the qubit device. A cantilevered conductor is formed in aportion of the housing and separated from the shunt capacitor by aspacing, the cantilevered conductor being connected to a ground andcapable of movement within the housing. An adjustment mechanism isconfigured to deflect the cantilevered conductor to tune a qubitfrequency for the qubit device.

A method for adjusting qubit frequency includes measuring a qubitfrequency of a qubit device; computing an adjustment to achieve adesired qubit frequency; adjusting a space between a cantileveredconductor and a shunt capacitor of the qubit device; and verifying thatthe qubit frequency has been corrected by the adjusting of the space.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a qubit device in a housing orstructure with a cantilevered conductor over a shunt capacitor fortuning qubit frequency in accordance with the present principles;

FIG. 2 is a cross-sectional view of a qubit device in a housing orstructure with a cantilevered conductor adjusted using a mechanicaladjustment mechanism to control a space or shape of the cantileveredconductor over a shunt capacitor for tuning qubit frequency inaccordance with the present principles;

FIG. 3 is a cross-sectional view of a qubit device in a housing orstructure with a cantilevered conductor adjusted using an electricaladjustment mechanism to control a space or shape of the cantileveredconductor over a shunt capacitor for tuning qubit frequency inaccordance with the present principles;

FIG. 4 is a cross-sectional view of a qubit device in a housing orstructure with two cantilevered conductors over a shunt capacitor fortuning qubit frequency in accordance with the present principles; and

FIG. 5 is a block/flow diagram showing a method for adjusting a qubitfrequency in accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, methods and systems areprovided that adjust each qubit frequency to eliminate undesirablespreads of frequencies. In one embodiment, qubit frequency is adjustedby altering shunt capacitance(s) to ground. This can be achieved byproviding an adjustable capacitance between one or both of the qubitshunt capacitor pads to ground. The qubit frequency can be adjusted bychanging the qubit capacitance.

Qubit capacitance may be adjusted using a relationship such as qubitfrequency→ω+ωΔC/2C. So to change the frequency by 5%, the capacitance ischanged by 10% which, in one example, can amount to about 6 fF. Theimpact of such a difference in capacitance (6 fF) on the qubit frequencyonly results in a 30 MHz change in the anharmonicity. This will notlimit the qubit performance and will tune qubit frequency with onlyminor effects on the qubit anharmonicity. In useful embodiments, amechanical cantilever is coated with metal and spaced apart from theshunt capacitor pads. The spacing between the cantilever and the qubitshunt capacitor is adjusted to tune the qubit frequency. Spacing may beadjusted in a plurality of ways, e.g., by mechanical or electricalmethods. In one embodiment, accurate adjustment of the qubit is providedto ensure a qubit frequency remains with an acceptable tolerance for oneof a plurality of distinct frequency values needed for quantumcomputing.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. Thechips are packaged in a manner consistent with the cryogenic environmentneeded by the qubits and then would be assembled into a larger systemforming a quantum computer or other device employing the properties ofthe qubit structures.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a schematic diagram shows aqubit 10 having a Josephson junction 12. The qubit 10 represents a statedevice for a superconducting quantum computer or other device. A shuntcapacitor 18 is coupled to superconducting electrodes of the Josephsonjunction 12. The shunt capacitor 18 in this embodiment includes a planarcapacitor having pads 20, 22.

Supercurrent (I_(s)) through a Josephson junction (JJ) is given byI_(s)=I_(c) sin(φ), where φ is the phase difference of superconductingwave functions of the two electrodes 20, 22, i.e. the Josephson phase.Critical current (I_(c)) is the maximum supercurrent that can flowthrough the Josephson junction 12. This critical current gives rise to anon-linear inductance L given by

$L = \frac{\Phi_{0}}{2\pi\; I_{c}}$where Φ₀ is the flux quantum. The qubit is a microwave oscillator with acharacteristic frequency for the 0-1 transition given by ω=1√{squareroot over (LC)}, where L is the inductance of the Josephson junction 12and C is the shunt capacitance defined by plates 20 and 22.

The qubit 10 has a characteristic frequency determined by a capacitancebetween one or more of the pads 20, 22 and an inductance of theJosephson junction 12. With the addition of a cantilevered conductor 14in proximity of the pads 20, 22, the qubit frequency can be adjusted. Bycontrolling a position of the cantilevered conductor 14, the capacitancefrom one or both qubit capacitor pads 20 and 22 to ground can becontrolled, e.g., by bending the cantilevered conductor 14. Theeffective shunt capacitance of the qubit depends on the combination ofcapacitances between pads 20 and 22, plus capacitance of each pad 20 and22 to ground. Therefore, by adjusting the latter two capacitance terms,the effective shunt capacitance of the qubit (and therefore qubitfrequency) is adjusted as well. The cantilevered conductor 14 mayinclude a flexible material, such as a polymer or a metal. If a polymeris employed, the polymer is conductive or may be coated in metal if thepolymer is formed from dielectric material.

The cantilevered conductor 14 is grounded and may be suspended in astructure or housing 15 using semiconductor processing. For example, thequbit 10 may be formed on a substrate 30. A dielectric layer or layers32 are deposited. A metal layer (14) is deposited on a support material34. The support material 34 may bond to the metal (14) and may include,e.g., an oxide or a nitride. The metal (14) is patterned. A sacrificialmaterial may be deposited in a cavity 38. The sacrificial material mayinclude, e.g., polysilicon or amorphous silicon. Then, anotherdielectric layer 36 is formed over the qubit 10 and cantileveredconductor 14. The sacrificial material is removed by a selective etch toexpose the metal 14 and the qubit 10 in the cavity 38. Other processingmethods may be employed to isolate the qubits 10 and the cantileveredconductor 14. The cantilevered conductor 14 is connected to groundpotential (connection through the layer 36 (not shown)).

In one conservative example, a change the capacitance of 6 fF is neededfor a qubit. To get a 6 fF change in a 200 micron×100 micron pad (20,22) in the air, 10 microns away, a deflection of the cantileveredconductor 14 of about 5 microns is needed. Assuming a control level of1/10 micron, a capacitance error of about 0.17 fF is achieved. In otherwords, ΔC=C Δx/d where d is the distance to the pad (20, 22) from thecantilevered conductor 14 and Δx is the change in d. If noise is assumedon this distance, then |ΔC|/C=|Δx/d| so the frequency will be|Δω|/ω=|ωx/(2d)|. If we want kHz range frequency changes, we need tomake sure Δx is controlled to less than a 1 micron change. Thevibrational mode of these devices tends to be much less than this.

In one embodiment, the as-formed metal 14 may provide the neededadjustment to the capacitance to achieve the desired qubit frequency.However, in accordance with the present principles, an adjustmentmechanism is employed to tune the metal 14 to achieve a desired qubitfrequency. The adjustment mechanism may include mechanical, electrical,electro-mechanical, magnetic or any other mechanism configured to changea spacing or shape of the cantilevered conductor.

Referring to FIG. 2, one embodiment employs the adjustment mechanism asa mechanical adjustment for the cantilevered conductor 14 (FIG. 1). Amechanical cantilever 102 may be metal or coated with metal 104. Themetal 104 may include Al, Cu, Ni, Pt, Au, or other conductive materials.The metal 104 is grounded. A spacing 106 between the cantilever 102 andthe qubit shunt capacitor 18 is adjusted to tune the qubit frequency, asdescribed. The spacing 106 may be adjusted using an actuation device108. The actuation device 108 may include a screw, a cam, a solenoidactuated arm, a piezoelectric actuator, or equivalents.

Referring to FIG. 3, another embodiment of the adjustment mechanismemploys an electrical adjustment for the cantilevered conductor 14 (FIG.1). The mechanical cantilever 102 may be metal or coated with metal 104.The metal 104 may include Al, Cu, Ni, Pt, Au, or other conductivematerials and preferably superconducting materials, such as Al, Nb, etc.The metal 104 is grounded. A spacing 106 between the cantilever 102 andthe qubit shunt capacitor 18 is adjusted to tune the qubit frequency, asdescribed. The spacing 106 may be adjusted using a capacitive forceactuation device 110. The actuation device 110 may include two plates112, 114. One plate 112 is fixed to structure or layer 116, and theother plate 114 is coupled to the cantilever 102. A voltage source 118is coupled to the plates to generate charge build up on the plates 112and 114. The charge build up can be controlled, such that the amount ofcharge causes the spacing 106 to change, which in turn tunes the qubitfrequency of the qubit 10. Other methods may also be employed, e.g.,using magnetic fields produced by inductors to deflect the cantilever.

In accordance with one embodiment, an estimate of capacitive force maybe provided. The area of the capacitor formed by plates 112 and 114 maybe, e.g., 200×100 microns with a gap between the plates of, e.g., 10microns. The capacitance=17 fF. The force is given by (½ CV²)/Gap. Forthe voltage source output of 1 volt, the force is 1.7×10⁻⁹ N. The forceconstant is 0.01 N/m and the displacement is 1.7×10⁻⁷ m (or 0.2microns). In this example, displacements on the order tenths of micronsare achieved with one volt. This is sufficient to change qubit shunt toground and alter the qubit frequency.

Referring to FIG. 4, other embodiments may employ more than onecantilevered ground metal. A system 200 includes two conductivecantilevers 210 and 212. The cantilevers 210, 212 may be deflected usingmechanical, electrical or other methods. The cantilevers 210, 212 may bedeflected together or separately and may be deflected by a same amountor different amounts, as needed.

In accordance with the present principles, to adjust each qubitfrequency to eliminate undesirable spreads of frequencies, adjustment ofthe qubit is provided to ensure a qubit frequency remains within anacceptable tolerance for one of the distinct frequency values needed forquantum computing. To control the qubit frequencies, adjustment to thecapacitance of the qubit may be provided using cantilevered conductors.

It should be understood that FIGS. 1-4 illustratively depict a Josephsonjunction as the qubit device; however, other devices may be employed. Inaddition, while the shunt capacitor is the structure being adjusted, itis understood that adjustments may be made on other structures to adjustthe frequency of the qubit device.

Referring to FIG. 5, a block/flow diagram shows a method for adjustingqubit frequency in accordance with the present principles. In somealternative implementations, the functions noted in the blocks may occurout of the order noted in the figures. For example, two blocks shown insuccession may, in fact, be executed substantially concurrently, or theblocks may sometimes be executed in the reverse order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

In block 102, a qubit device or a device including qubit devices (e.g.,integrated circuit) is fabricated by known processing techniques. Inblock 104, the qubit device (or qubit) is at least partiallyencapsulated in a housing. The housing isolates the qubit device andprovides a fixed position to mount a conductive cantilever. In block106, the qubit frequency (distribution) is measured. This may includemeasuring a relevant metric including qubit frequency, critical currentand/or other properties.

In block 108, a correction is determined and needed modifications tocorrect the qubit frequency are made. Computing the correction includesan accurate calculation of the amount of adjustment to change thespacing or shape of the cantilevered conductor to achieve the desiredqubit frequency, while maintaining other qubit parameters, such as,anharmonicity.

In block 110, a space between the cantilevered conductor and a shuntcapacitor of the qubit device is adjusted in accordance with thefrequency correction. This may include adjusting one or a plurality ofcantilevered conductors.

In block 112, adjusting the space may include adjusting a mechanicaladjustment to tune the qubit frequency. The mechanical adjustment mayinclude a screw, a cam, a solenoid actuated arm, a piezoelectricactuator, etc. In block 114, adjusting the space may include adjustingan electrical adjustment to tune the qubit frequency. The electricaladjustment may include capacitive plates, wherein one plate is fixed andanother is disposed on the cantilevered conductor.

In block 116, a verification measurement may be performed to make surethat the adjustment of the frequency has its intended effect. Adetermination is made as to whether the frequency is correct. Thecorrect frequency may include one or more distinct operating frequenciesof the qubit (e.g., 5 or more). In addition, the frequency correctionneeds to be within an acceptable tolerance of the distinct distributionsof frequency, e.g., less than about 5% and more preferably less thanabout 1%. In block 118, if the frequency is correct, then the processends in block 120. Otherwise, the process continues by returning toblock 106. The process is repeated as necessary.

Having described preferred embodiments mechanically tunablesuperconducting qubit (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

The invention claimed is:
 1. A system for adjusting qubit frequency,comprising: a qubit device including a Josephson junction and a shuntcapacitor coupled to electrodes of the Josephson junction; acantilevered conductor separated from the shunt capacitor by a spacing;and an adjustment mechanism configured to deflect the cantileveredconductor to tune a qubit frequency for the qubit device.
 2. The systemas recited in claim 1, wherein the shunt capacitor includes pads and thecantilevered conductor is disposed over one or more of the pads.
 3. Thesystem as recited in claim 1, wherein the adjustment mechanism includesa mechanical adjustment.
 4. The system as recited in claim 3, whereinthe mechanical adjustment includes one of a screw, a cam, a solenoidactuated arm or a piezoelectric actuator.
 5. The system as recited inclaim 1, wherein the adjustment mechanism includes an electricaladjustment.
 6. The system as recited in claim 5, wherein the electricaladjustment includes capacitive plates, wherein one plate is fixed andanother is disposed on the cantilevered conductor.
 7. The system asrecited in claim 1, wherein the cantilevered conductor includes a metalor a metal coated polymeric material.
 8. The system as recited in claim1, wherein the cantilevered conductor includes a plurality ofcantilevered conductors configured to be spaced apart from differentportions of the shunt capacitor.
 9. A system for adjusting qubitfrequency, comprising: a qubit device including a Josephson junction anda shunt capacitor coupled to electrodes of the Josephson junction; ahousing at least partially encapsulating the qubit device; acantilevered conductor formed in a portion of the housing and separatedfrom the shunt capacitor by a spacing, the cantilevered conductor beingconnected to a ground and capable of movement within the housing; and anadjustment mechanism configured to deflect the cantilevered conductor totune a qubit frequency for the qubit device.
 10. The system as recitedin claim 9, wherein the shunt capacitor includes pads and thecantilevered conductor is disposed over one or more of the pads.
 11. Thesystem as recited in claim 9, wherein the adjustment mechanism includesa mechanical adjustment.
 12. The system as recited in claim 11, whereinthe mechanical adjustment includes one of a screw, a cam, a solenoidactuated arm or a piezoelectric actuator.
 13. The system as recited inclaim 9, wherein the adjustment mechanism includes an electricaladjustment.
 14. The system as recited in claim 13, wherein theelectrical adjustment includes capacitive plates, wherein one plate isfixed to the housing and another is disposed on the cantileveredconductor.
 15. The system as recited in claim 9, wherein thecantilevered conductor includes a metal or a metal coated polymericmaterial.
 16. The system as recited in claim 9, wherein the cantileveredconductor includes a plurality of cantilevered conductors configured tobe spaced apart from different portions of the shunt capacitor.
 17. Amethod for adjusting qubit frequency, comprising: measuring a qubitfrequency of a qubit device; computing an adjustment to achieve adesired qubit frequency; adjusting a space between a cantileveredconductor and a shunt capacitor of the qubit device; and verifying thatthe qubit frequency has been corrected by the adjusting of the space.18. The method as recited in claim 17, further comprising, repeating thesteps until the qubit frequency has been corrected.
 19. The method asrecited in claim 17, wherein adjusting the space includes adjusting amechanical adjustment to tune the qubit frequency.
 20. The method asrecited in claim 17, wherein adjusting the space includes adjusting anelectrical adjustment to tune the qubit frequency.